I've fished for the famous "ghost of the flats" on Christmas Island, Kiribata(Pacific Ocean, not the Indian Ocean one).
Size 6 Crazy charlies worked there, but I was only fishing in ankle deep water.
How deep is the water you're fishing? I think it is important to get the fly onto the bottom. Heavy clouser on fast sinking line(even LC13).
Have you tried to burley them up to get them feeding hard?
Keep flogging the water to a foam. And throw every fly in your box at them.
Best of luck.
Wes